Wireless telecommunication system, transmitter and receiver

ABSTRACT

In a wireless telecommunication system for transmitting a plurality of streams from a base transmission station comprising a plurality of antennas to a mobile station comprising a plurality of antennas, the base transmission station respectively transmits, by way of a plurality of the antennas, each stream obtained by dividing data to be transmitted. The mobile station respectively receives each stream by utilizing the plurality of antennas. A measurement unit respectively measures a reception quality or received power from each antenna relating to a stream. A change over unit carries out a handover for each stream according to a measurement result by the measurement unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an MIMO (multiple input multipleoutput) transmission technology for transmitting a plurality of streamsby way of a plurality of antennas.

2. Description of the Related Art

In a wireless telecommunication system, mobile terminals (i.e.,terminals) are enabled to continue a telecommunication if a receivedpower, et cetera, is reduced by selectively changing a connected basestation over to an optimal one based on a received power, a receptionquality, et cetera, from surrounding base stations (e.g., refer topatent documents 1, 2, 3 and 4; and a non-patent document 1). An exampleof a wireless telecommunication system is the recently propagated W-CDMA(Wideband Code Division Multiple Access) system.

FIG. 1 exemplifies a configuration of a telecommunication systemadopting the W-CDMA system. The terminal (i.e., a mobile station, MS)receives signals from a plurality of base transmission stations (BTS)BTS_(a), BTS_(b), BTS_(c) through BTS_(n). The mobile station selects abase transmission station with the largest received power as the optimalbase transmission station for the telecommunication from among theplurality thereof to connect to a network via the selected basetransmission station. If a received power from a nearby basetransmission station (i.e., adjacent base transmission station) becomeslarger than that from the connected base transmission station as aresult of the mobile station moving for example, a handover, i.e.,changing the connected base transmission station to the nearby one, iscarried out for a continued telecommunication. A handover is alsocarried out according to the telecommunication environment in the caseof transmitting and receiving a plurality of data streams simultaneouslybetween a base transmission station and mobile station by utilizing theMIMO technique. The following description is of a handover processingutilizing the MIMO technique in a wireless telecommunication systemadopting the HSDPA (high speed downlink packet access) as one standardspecification of W-CDMA.

FIG. 2 shows an overview of a handover processing according to theconventional technique; illustrating a handover processing in a downlinktransmission under the HSDPA system 100, that is, in a data transmissionfrom the base transmission stations to a mobile station. In the exampleof FIG. 2, a 2 by 2 antenna configuration is structured between a basetransmission station and a mobile station.

Prior to a handover, a mobile station 102 receives a data stream via abase transmission station 103A as shown by FIG. 2(a). As the receivedpower from a base transmission station 103B becomes larger than that ofthe base transmission station 103A at the mobile station 102, a handoverprocessing changes the base transmission station 103A to the basetransmission station 103B with a larger received power, as the basetransmission station for transmitting a data stream over, as shown byFIG. 2(b). In this event, the two data streams transmitted from the basetransmission station 103A are simultaneously changed over. The handoverprocessing results in the mobile station 102 receiving two data streamsvia the base transmission station 103B.

FIGS. 3, 4A and 4B exemplify respective configurations of a receivingapparatus and a transmission apparatus according to the conventionaltechnique. Here, the mobile station 102 is a receiving apparatus, whilethe base transmission station 103 is the transmission apparatus. And inthe examples of FIGS. 3, 4A and 4B, the mobile station 102 and the basetransmission station 103 comprise, respectively, three antennas, fortransmission and reception, thereby configured to transmit and receivethree data streams simultaneously. Incidentally, antennas (i.e., Tx₁through Tx₃; and Rx₁ through Rx₃), utilized for transmitting the datastream from the base transmission station 103 to the mobile station 102,are delineated separately from antennas (i.e., Tx₀ and Rx₀) fortransmitting a signal from the mobile station 102 to the basetransmission station 103, for ease of viewing of FIGS. 3, 4A and 4B. Buteach antenna is used both for transmitting and receiving signals in theactual structure. That is, the antenna Tx₀ comprised by the mobilestation 102 is achieved by a discretionary one or a plurality of theantennas Rx₁ through Rx₃, and likewise the antenna Rx₀ comprised by thebase transmission station 103 is achieved by a discretionary one or aplurality of the antennas Tx₁ through Tx₃. Note that this also appliesto those drawings herein where the transmission and reception antennasare separately delineated in the following descriptions.

The conventional mobile station 102 as shown by FIG. 3 comprises threeantennas Rx₁, Rx₂ and Rx₃, and the corresponding receiver units 11A,111B and 111C. Received power measurement units 112A, 112B and 112Cmeasure the received power from each base transmission station 103 atthe respective receiver units 111A, 111B and 111C. A handover judgmentunit 113 calculates the total received power, i.e., a sum of thereceived power at each base transmission station based on themeasurement result at each received power measurement unit 112 andjudges whether a handover is to be carried out or not.

In the conventional base transmission station 103 as shown by FIG. 4A,the antenna Rx₀ receives handover control information transmitted fromthe antenna Tx₀ of the mobile station 102. The received information isprovided to a handover control signal extraction unit 131 by way of thereceiver unit. The handover control signal is transmitted to an upperlevel control station, that is, a radio network controller (RNC) in theHSDPA system 100.

FIG. 4B shows a configuration, relating to a handover processing, of anRNC as the upper level station of a base transmission station 103. As ahandover signal is transmitted from the base transmission station 103 tothe RNC, a handover control unit thereof determines a timing forcarrying out a handover. A handover control unit 132 of the basetransmission station 103 carries out a handover by receiving anotification from the RNC.

FIG. 5 shows a process procedure of a handover in the conventionalsystem. The first step is to measure a received power from each basetransmission station (S101), compare the total received power from thesurrounding base transmission station of the mobile station 102 (S102),judge whether or not a handover is to be carried out based on thecomparison result and, if it is judged that a handover is to be carriedout, determine which base transmission station is to be handed over to(S103).

FIG. 6 is a flow chart showing a definite measurement process of areceived power as a part of the conventional handover processing. In theexample shown by FIG. 6, the n-number of base transmission stationsexist in the surrounding area of the mobile station, and each terminalstation is equipped by the m-number of antennas respectively. Meanwhile,it is defined that a received power of a signal from the basetransmission station of a base transmission station number j is P_(j)and that the power received by the antenna A_(x) is P_(rx).

In the measurement process of a received power, initialize the antennanumber, the base transmission station number and the received power fromeach base transmission station (S111), increment the base transmissionstation number j and the antenna number i, by one, respectively (S112and S113) and add the received power P_(rx) when receiving a signal fromeach base transmission station via each antenna Ai and acquire the sumof the received power (S114, S115 and S116). The sum of received powersis acquired for the n-number of base transmission stations, respectively(S117).

In the conventional handover processing, the necessity of a handover isjudged based on the sum of the received powers and when the basetransmission stations are changed over from the current basetransmission station to a target base transmission station fortransmitting a data stream, the data stream is transmitted via adifferent base transmission station pre- and post- the handover, asshown by FIGS. 5 and 6.

FIG. 7 illustrates data transmission pre- and post- a handover accordingto the conventional technique. The handover is defined as the mobilestation 102 moving from under the base transmission station BTS₁ to theBTS_(h). As shown by FIG. 7(a), the base transmission station BTS₁transmits data prior to a handover and, when the upper level controlstation, i.e., an RNC, carries out a handover at a prescribed timing, itcuts off the path between the RNC and base transmission station BTS, andinstead establishes a path between the RNC and another base transmissionstation BTS_(h). As shown by FIG. 7(b), the data is then transmitted viathe post-handover base transmission station BTS_(h), leaving dataaccumulated in the base transmission station BTS₁ prior to the handoverun-transmitted.

If the accumulated data in the base transmission station BTS, as shownby FIG. 7 is left un-transmitted, the mobile station 102 is unable toreceive all the data. In order to prevent such a loss of data fromoccurring, a data transfer or a retransmission has been carried out inthe conventional technique.

FIGS. 8 through 11 describe the processing for preventing a data loss ata handover occurrence of the conventional system. In FIGS. 8 through 11,definitions for designations are as follows: a current base transmissionstation (i.e., the station which hands a terminal over): BTS_(a), atarget base transmission station (i.e., the station which a terminal ishanded over to) BTS_(b), the data delivered to the BTS_(a), and BTS_(b)are data A and data B, respectively.

FIG. 8 describes a state of a buffer at each base transmission stationprior to a handover. A radio network controller RNC, i.e., the upperlevel control station of a base transmission station, transmits data tothe mobile station via the connected BTS_(a). The data A is accumulatedin the buffer of the BTS_(a).

FIG. 9 describes a state of the buffer at each base transmission stationimmediately after carrying out a handover in the case of retransmittingdata after the handover. After carrying out a handover, the mobilestation 102 connects itself to a network via the BTS_(b). The buffer ofthe BTS_(b) accumulates the data B after carrying out a handover.Meanwhile, the data A which has been delivered to the BTS_(a) beforecarrying out the handover ends up remaining in the buffer thereof.

FIG. 10 describes a data retransmission processing by the upper levelcontrol station. As shown by FIG. 9, the data A is the data transmittedto the current base transmission station BTS_(a) prior to the handoverand accumulated in the buffer. The RNC, i.e., the upper level controlstation, receiving a request from a mobile station 102, retransmits thedata A accumulated in the BTS_(a) to the BTS_(b), that is, the targetbase transmission station, while the data A in the BTS_(a) is discarded.In the BTS_(b), a signal exchange between the mobile station 102 and RNCis conducted for a retransmission control prior to a retransmissionprocessing of the data A, followed by retransmitting the data A. Anexchange of a retransmission control signal and the actualretransmission processing are time consuming.

FIG. 11 describes a state of a buffer at each base transmission stationimmediately after carrying out a handover in the case of transferringdata after the handover. This method transfers, to the BTS_(b) via theRNC, data A accumulated in a buffer of the BTS_(a), as a result of ahandover. Similarly, for such a transfer, an exchange of a controlsignal and actual transfer processing are time consuming as with thedata retransmission processing shown by FIG. 10.

As described by referring to FIGS. 8 through 11, since a plurality ofdata streams simultaneously transmitted are changed over at the sametime in the conventional technique, a data loss occurs in a mobilestation 102 unless the data accumulated in the buffer of a current basetransmission station is retransmitted or transferred to the mobilestation 102. There is a problem of a handover process being timeconsuming as much as the processing of retransmitting or transferringthe data taking time, hence resulting in a decreased transmission speed.

Meanwhile, in a system for wirelessly and simultaneously transmittingmutually independent data streams respectively from a plurality oftransmission systems by using the same frequency, as a MIMO relatedtechnique, there is a technique for wirelessly transmitting atransmission data stream simultaneously by using the same frequency,respectively, from a plurality of transmission systems, followingchanging over from a mutually independent plurality of data streams to aplurality of sub-streams, if a received power of a mobile station (or areception quality) assumes a prescribed threshold value or less.According to such a technique, a MIMO transmission is carried out if amobile station exists in the neighborhood of a base transmission stationwith a relatively large received power, while a diversity transmissionis performed if a mobile station exists in an area of a relatively lowreceived power. In the neighborhood of the two areas, a changeoverbetween the MIMO transmission and the diversity transmission isconducted.

[Patent document 1] laid-open Japanese patent application publicationNo. 2004-72624

[Patent document 2] laid-open Japanese patent application publicationNo. 2003-338781

[Patent document 3) laid-open Japanese patent application publicationNo. 2004-229088

[Patent document 4] laid-open Japanese patent application publicationNo. 2005-509565

[Non-patent document 1] 3GPP Specification: 25.211 Re1-5, Version 5.7.0,[online], Aug. 2, 2005, 3GPP, searched on Aug. 3, 2005, Internet <URL:http://www.3gpp.org/ftp/Specs/html-info/25211.htm>

SUMMARY OF THE INVENTION

The purpose of the present invention is to shorten the time required fora handover, thereby preventing a decreased transmission speed at thetime of carrying out a handover.

In order to solve the above described problem, the present inventionprovides a wireless telecommunication system for transmitting aplurality of streams from a transmission apparatus comprising aplurality of antennas to a receiving apparatus, comprising atransmission unit, being equipped in the transmission apparatus, fortransmitting a plurality of streams, a receiving unit, being equipped inthe receiving apparatus, for receiving individually each streamtransmitted from the transmission unit, a measurement unit for measuringrespectively a reception quality or a received power from thetransmission unit relating to each stream, and a changeover unit forcarrying out a handover for each of the streams according to ameasurement result of the measurement unit.

Each receiving unit measures a reception quality and received powertransmitted from a transmission unit for each stream. A handover iscarried out in the unit of a stream if the result of a measurementindicates that it is preferable that a handover is carried out. Adiscard, transfer or retransmission process for data at the current basestation is no longer required, hence shortening the time required for adata transmission.

Or, it may further comprise a judgment unit for judging whether or notit is necessary to carry out a handover by comparing a reception qualityor received power relating to a stream from a first transmission unit ina first transmission apparatus being used for a telecommunication with areception quality or received power, respectively, relating to a streamfrom a second transmission unit in a second transmission apparatus,wherein said changeover unit changes a transmission unit used for atelecommunication for a stream from the first transmission unit to thesecond transmission unit, if it is judged as necessary to carry out ahandover.

Furthermore, it may be configured in such a manner that said judgmentunit judges that it is necessary to carry out a handover if a receptionquality or received power relating to a stream from said secondtransmission unit assumes a predetermined threshold value or greater,respectively, as compared to a reception quality or received powerrelating to the stream from said first transmission unit.

And the present invention provides a transmitter comprising a pluralityof antennas for transmitting a plurality of streams, comprising atransmission unit for transmitting a plurality of streams, and achangeover unit for carrying out a handover for each stream according torespective measurement results of a reception quality or received powerfrom the transmission unit relating to each stream.

Moreover, the present invention provides a receiver for receiving aplurality of data streams, comprising a receiving unit for receiving aplurality of streams transmitted from a first and a second transmitter,a measurement unit for respectively measuring a reception quality orreceived power from the transmitter relating to each stream, and asignal reproduction unit for reproducing a signal from one or morestreams transmitted from the first transmitter and one or more streamstransmitted from the second transmitter if it is judged that it isnecessary to carry out a handover to the second transmitter, for afraction of streams among a plurality thereof transmitted from the firsttransmitter, based on a measurement result of the measurement unit.

According to the present invention, once a handover is carried out foreach of the data stream, there is no need for a current transmissionapparatus to discard, retransmit or transfer data, and therefore it ispossible to shorten the time required for the handover processing.Shortening the time required for the handover processing prevents areduced transmission speed at the time of carrying out a handover.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 exemplifies a configuration of a telecommunication systemadopting the W-CDMA system;

FIG. 2 shows a summary of a handover processing according to theconventional technique;

FIG. 3 exemplifies a comprisal of a mobile station according to theconventional technique;

FIG. 4A exemplifies a comprisal of a base transmission station accordingto the conventional technique;

FIG. 4B exemplifies a comprisal of a radio network controller accordingto the conventional technique;

FIG. 5 shows a process procedure of a handover in a conventional system;

FIG. 6 is a flow chart showing a definite measurement processing of areceived power as a part of the conventional handover processing;

FIG. 7 illustrates data transmission pre- and post-handover according tothe conventional technique;

FIG. 8 describes a state of a buffer at each base transmission stationprior to a handover;

FIG. 9 describes a state of the buffer at each base transmission stationimmediately after carrying out a handover in the case of retransmittingdata after the handover;

FIG. 10 describes a data retransmission processing by an upper levelcontrol station;

FIG. 11 describes a state of a buffer at each base transmission stationimmediately after carrying out a handover in the case of transferringdata after the handover;

FIG. 12 is a conceptual diagram of a handover method according to thepresent invention;

FIG. 13 shows a comprisal of a mobile station carrying out a handoveraccording to a first embodiment;

FIG. 14 illustrates a transmission and reception of a data streamaccording to the first embodiment;

FIG. 15 shows a comprisal of a base transmission station carrying out ahandover method according to the first embodiment;

FIG. 16 illustrates a handover method according to the first embodiment;

FIG. 17 is a flow chart of the processing of judging a necessity ofhandover;

FIG. 18 is a control sequence chart (part 1) of a handover judgmentprocessing;

FIG. 19 exemplifies a data structure of handover information;

FIG. 20A is an actual example of handover information (part 1);

FIG. 20B is an actual example of handover information (part 2);

FIG. 20C is an actual example of handover information (part 3);

FIG. 20D is an actual example of handover information (part 4);

FIG. 21 is a flow chart of transmission processing for accumulated datain a current base transmission station;

FIG. 22 illustrates a state of accumulated data at the base transmissionstation after carrying out a handover;

FIG. 23 is a control sequence chart (part 2) of a handover judgmentprocessing;

FIG. 24 shows a comprisal of a mobile station carrying out a handoveraccording to a second embodiment;

FIG. 25 is a flow chart of a handover processing according to the secondembodiment;

FIG. 26 is a flow chart of a handover processing according to a thirdembodiment;

FIG. 27A exemplifies a selected target base transmission station;

FIG. 27B exemplifies a received power from a target base transmissionstation for each stream;

FIG. 28 is a flow chart of a handover processing according to a fourthembodiment;

FIG. 29 illustrates a handover method according to a fifth embodiment;

FIG. 30 is a conceptual diagram of a system configuration according to asixth embodiment; and

FIG. 31 is a flowchart of a transmission method changeover processingaccording to the sixth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a detailed description of the preferred embodiment ofthe present invention referring to the drawings.

First Embodiment

FIG. 12 is a conceptual diagram of a handover method according to thepresent invention. A wireless telecommunication system 1 using a MIMOtransmission technique which adopts HSDPA (high speed downlink packetaccess) as one standard specification of W-CDMA comprises basetransmission stations BTS 3 (BTS_(3a) and BTS_(3b)) and a mobile station(MS) 2. As shown by FIG. 12(a), the mobile station 2 receives aplurality of data streams via the base transmission station BTS_(3a)prior to a handover. With regard to a received power for the same datastream, if a received power from another base transmission station,which the own mobile station 2 is not connected to, is larger than thatfrom the connected base transmission station, a handover is carried outfor the aforementioned data stream. As shown by FIG. 12(b), a datastream after the handover being carried out is transmitted to the mobilestation 2 via a base transmission station BTS_(3b). The data streams forwhich a handover is not carried out are transmitted to the mobilestation 2 via the base transmission station BTS_(3a) as before. That is,the mobile station 2 receives one or more data streams from the basetransmission station BTS_(3a) and at the same time receives one or moredata streams from the base transmission station BTS_(3b).

FIG. 13 shows a comprisal of a mobile station 2 carrying out a handoveraccording to the first embodiment. In the example shown by FIG. 13, acomprisal of a three by three (3×3) antennas is shown as an example of awireless telecommunication system of m by n (m×n) antennas (where m andn are integer).

The mobile station 2 shown by FIG. 13 includes a plurality of antennasRx (Rx₁, Rx₂ and Rx₃), a plurality of receiver units 21 (21A, 21B and21C), a signal separation synthesis unit 24, a decode process unit 25, astream received power measurement unit 22, a handover judgment unit 23and a transmission unit 26.

The plurality of receiver units 21 are interfaces for processing signalsreceived by way of the respective antennas Rx. The signal separationsynthesis unit 24 reproduces a plurality of data streams from signalsreceived from the base transmission station 3 by way of the receivingunits 21 to provide them to the decode process unit 25 and the streamreceived power measurement unit 22. In this event, only a pilot signalmay be transmitted to the stream received power measurement unit 22. Thedecode process unit 25 decodes the reproduced data streams. The streamreceived power measurement unit 22 measures a received power of the eachstream (i.e., a pilot signal specifically) reproduced by the signalseparation synthesis unit 24. In this event, the stream received powermeasurement unit 22 measures a received power of the each streamreceived from the currently connected base transmission station and thatof each stream from other base transmission stations positioned in thesurrounding area of the mobile station 2. The handover judgment unit 23judges a necessity of a handover based on the measurement result of thestream received power measurement unit 22. The transmission unit 26 isan interface for transmitting a message such as a handover request byway of the antenna Tx₀ in the present embodiment.

In the conventional wireless telecommunication system, a sum of thereceived power of a plurality of streams (that is, the total receivedpower) is measured for each base transmission station, and a necessityof a handover has been judged according to the total received power asdescribed by referring to FIGS. 2A through 6. Comparably, in thewireless telecommunication system 1 according to the present embodiment,the received power is measured for each stream and a necessity of ahandover is judged according to the result.

FIG. 14 illustrates a transmission and reception of a data streamaccording to the present embodiment. Under the control of the wirelesstelecommunication system 1, the mobile station 2 receives a data streamfrom a plurality of base transmission stations 3, that is, from two basetransmission stations BTS_(3a) and BTS_(3b) in the system comprisalshown by FIG. 13. In an m by n (m×n) wireless telecommunication system,the base transmission station 3, i.e., the transmitter side apparatusfor data, divides data to be transmitted and transmits a data stream byway of the m-number of antennas. The mobile station 2, i.e., thereceiving side apparatus for data, comprises the n-number of antennasand receives the data stream by way thereof. A channel response matrixused for separating a received signal received from the basetransmission station BTS_(3a) at the mobile station 2 into each datastream is as follows: $\begin{matrix}{H = \begin{bmatrix}a_{11} & a_{12} & \ldots & a_{1m} \\a_{21} & a_{22} & \ldots & a_{2m} \\\vdots & \vdots & ⋰ & \vdots \\a_{n\quad 1} & a_{n\quad 2} & \ldots & a_{mn}\end{bmatrix}} & (1)\end{matrix}$

The signal separation synthesis unit 24 separates, by using the matrixshown by the expression (1) the data stream received via the basetransmission station BTS_(3a). The separated signal is provided to thedecode process unit 25 to obtain a decoded signal.

Note here that the mobile station 2 is configured to determine a channelresponse matrix between the base transmission station BTS_(3a) andmobile station 2 in advance by utilizing a pilot signal transmitted fromthe base transmission station BTS_(3a). And that the mobile station 2 isalso configured to determine a channel response matrix between a basetransmission station in the surrounding area (i.e., BTS_(3b) herein) andmobile station 2 by using a pilot signal transmitted from the basetransmission station BTS_(3b). The channel response matrix between thebase transmission station BTS_(3b) and mobile station 2 is shown by theexpression (2): $\begin{matrix}{H = \begin{bmatrix}b_{11} & b_{12} & \ldots & b_{1m} \\b_{21} & b_{22} & \ldots & b_{2m} \\\vdots & \vdots & ⋰ & \vdots \\b_{n\quad 1} & b_{n\quad 2} & \ldots & b_{nm}\end{bmatrix}} & (2)\end{matrix}$

Here, the assumption is that a received power of the stream transmittedfrom an antenna A₂ of the base transmission station BTS_(3a) is exceededby that of the stream transmitted from an antenna 2 of the basetransmission station BTS_(3b), resulting in the carrying out of ahandover according to the present invention as shown by FIG. 12. In thiscase, the channel response matrix used for separating into each stream,a signal received from the base transmission stations BTS_(3a) andBTS_(3b) at the mobile station 2 is expressed by: $\begin{matrix}{H = \begin{bmatrix}a_{11} & a_{12} & \ldots & a_{1m} \\b_{21} & b_{22} & \ldots & b_{2m} \\\vdots & \vdots & ⋰ & \vdots \\a_{n\quad 1} & a_{n\quad 2} & \ldots & a_{nm}\end{bmatrix}} & (3)\end{matrix}$

In the above expression (3), the matrix elements b₂₁, b₂₂ through b_(2m)in the second column express a propagation characteristic between thesecond antenna of the base transmission station BTS_(3b) and mobilestation 2.

Thus, replacement of the element, with that of the channel responsematrix of a target base transmission station, of the channel responsematrix relating to a data stream to be changed over carries out ahandover for each data stream according to the present invention.

FIG. 15 shows a comprisal of a base transmission station 3 carrying outa handover method according to the first embodiment, showing aconfiguration of three by three (3×3) antennas as in the case of FIG.13.

The base transmission station 3 shown by FIG. 15 comprises atransmission data unit 33, an S/P unit 34, a pilot generation unit 35, aplurality of transmission units 36 (i.e., 36A, 36B and 36C), a pluralityof antennas Tx (i.e., Tx₁, Tx₂ and Tx₃), a receiving unit 37, acontrol:signal decode unit 31 and a stream selection unit 32.

The transmission data unit 33 includes a memory for accumulating data tobe transmitted to the mobile station 2, which is received from an upperlevel control station (i.e., the RNC in the HSDPA system). The S/P unit34 parallelizes a serial signal received from the upper level controlstation to provide it to each of the transmission units 36. The pilotgeneration unit 35 generates a pilot signal corresponding to eachstream. The plurality of transmission units 36, i.e., 36A, 36B and 36Cin the example of FIG. 15, are interfaces for transmitting pilot signalsand data by way of the antennas Tx₁, Tx₂ and Tx₃, respectively. Thereceiving unit 37 is an interface for receiving a handover request fromthe mobile station 2, et cetera, shown by FIG. 13 by way of the antennaRx₀. The control signal decode unit 31 decodes a control signal given bythe receiving unit 37. The stream selection unit 32 selects a datastream to be changed over based on the decoded control signal andcontrols the S/P unit 34 and the pilot generation unit 35.

Let it be defined for the following description that the basetransmission station currently connected to the mobile station 2 is theBTS_(3a) and those base transmission stations positioned in thesurrounding area of the mobile station 2, other than the BTS_(3a), areBTS_(3b), BTS_(3c) through BTS_(3n). The stream received powermeasurement unit 22 comprised by the mobile station 2 measures thereceived power P_(bts3a), P_(bts3b) through P_(bts3n) of the respectivedata streams transmitted from the base transmission station 3. Thehandover judgment unit 23 compares the values of the received powerP_(bts3a), P_(bts3b) through P_(bts3n) for each data stream, therebyjudging a necessity of a handover. Note that a received power to bemeasured is defined as a received power which is measured based on apilot signal.

FIG. 16 illustrates a handover method according to the first embodiment.As shown by FIG. 16(a), all of three data streams, i.e., st₁, st₂ andst₃, are transmitted from the BTS_(3a) to the mobile station 2 prior toa handover. Among the three data streams, if the received power from theBTS_(3a) is exceeded by that from the BTS_(3b) for the data stream st₂,resulting in a judgment for carrying out a handover to the BTS_(3b)therefor, then the path for the data stream st₂ is changed over. Asshown by FIG. 16(b), the remaining two data streams, i.e., st₁ and st₃,are maintained for transmission from the BTS_(3a) to the mobile station2.

FIG. 17 is a flow chart of the processing for judging a necessity ofhandover. The processing shown by FIG. 17 is carried out only at thetime when the mobile station 2 configured as shown by FIG. 13 receives apilot signal transmitted from the base transmission station 3 at apredetermined time interval.

First, the step S1 sets a data stream St_(k) as the subject of judging anecessity of a handover, setting any one of the data streams st₁, st₂ orst₃ in this step according to the present embodiment. The step S2measures the received power P_(bts3a) _(—) _(stk) of a data streamSt_(k) received from the currently connected base transmission stationBTS_(3a). The step S3 measures the received powers P_(bts3b) _(—)_(stk), P_(bts3c) _(—) _(stk) through P_(bts3n) _(—) _(stk) of the datastream st_(k) relating to the nearby base transmission stationsBTS_(3b), BTS_(3c) through BTS₃. The step S4 calculates the maximumreceived power P_(bts3h) _(—) _(stk) from among the received power ofthe data stream st_(k) calculated in the step S3.

The step S5 compares the P_(bts3a) _(—) _(stk) from the currentlyconnected base transmission station BTS_(3a) with the maximum receivedpower P_(bts3h) _(—) _(stk) calculated in the step S4. If the maximumreceived power P_(bts3h) _(—) _(stk) is larger than the received powerP_(bts3a) _(—) _(stk,) the step S6 judges that there is a necessity ofcarrying out a handover, thus ending the processing. Conversely, if themaximum received power P_(bts3h) _(—) _(stk) is at the P_(bts3a) _(—)_(stk) or less, then the step S7 judges that there is no necessity ofcarrying out a handover, thus ending the processing. As for a datastream st_(k) for which the processing of the step S5 judges that thereis a necessity of a handover, then it is carried out so as to transmitto the mobile station 2 via the base transmission station BTS_(3h) withthe maximum received power.

The processing shown by FIG. 17 is carried out for each of all thesimultaneously transmitted data streams, that is, each of the three datastreams st1, st₂ and st₃ according to the present embodiment, andthereby the optimal base transmission station (i.e., being defined asBTS_(3i)) is determined for transmitting each data stream. Once it isdecided to carry out a handover for a certain data stream, a handovercontrol unit for controlling a handover at the RNC, et cetera,determines a target base transmission station BTS_(3i), transmits thedata transmitted from an upper level station thereto and stopstransmitting the data to the current base transmission station BTS_(3a).

Note that the present embodiment comprises the handover judgment unit 23in the mobile station 2 as shown by FIG. 13, but that the mobile station2 is not the only unit capable of judging a necessity of a handover. TheRNC or the base transmission station 3 may judge it based on ameasurement result of a received power for each data stream in themobile station 2. The following describes cases exemplifying the RNC andthe mobile station 2 judging a necessity of a handover to describe thecontrol processes for both cases in detail.

FIG. 18 is a control sequence chart of a handover judgment processing inthe case of the mobile station 2 judging a necessity of a handover. Theprocessing shown by FIG. 18 is only carried out at the time when themobile station 2 receives a pilot signal, transmitted from the basetransmission station at a predetermined time interval.

First in the mobile station 2, a received power is measured for eachdata stream. The measured received power for each data stream iscompared for judging a necessity of a handover. The measurement of areceived power for each data stream and the judgment for a necessity ofa handover based on the result of the measurement are the same as wasdescribed in reference to FIG. 17. Here, if the judgment is to carry outa handover, the mobile station 2 transmits a handover carry-outnotification message to the RNC, followed by transmitting handoverinformation to the RNC. Here, the handover information is informationabout a stream number, a target base transmission station, et cetera,for example, indicating which data stream is to be changed over to whichbase transmission station.

Having received the handover carry-out notification and the handoverinformation, the RNC transmits the handover information to the mobilestation 2. Here, the handover information transmitted from the RNC tothe mobile station 2 includes a stream number, target base transmissionstation information, current base transmission station information,handover timing information, et cetera, for example. Furthermore, theRNC determines and controls a trigger for carrying out a handover. TheRNC notifies the target base transmission station BTS_(3i), current basetransmission station BTS_(3a) and mobile station 2 of the handovertiming determined by the RNC. The handover is carried out according tothe handover timing notification from the RNC.

A handover start control processing is performed between the RNC and thebase transmission station, and the path of the data stream is changedover. In the handover processing, messages such as “line setup”, “lineconnection”, “line disconnection”, et cetera, are transmitted asnotifications to the target base transmission station BTS_(3i) and thecurrent base transmission station BTS_(3a). When completing thehandover, the mobile station 2 transmits a handover complete controlmessage to the target base transmission station, and the basetransmission station, having received the message, transmits a handovercomplete control message and ends the processing.

The handover information transmitted from the RNC to the mobile station2 in the sequence shown by FIG. 18 enables each telecommunicationapparatus to recognize information about a data stream or a basetransmission station to carry out a handover. The following descriptionis of information transmitted from the RNC to a base transmissionstation 3 and mobile station 2 which are under the RNC referring to FIG.19, and FIGS. 20A through 20D.

FIG. 19 exemplifies a data structure of handover information transmittedas a notification by the RNC to a lower level apparatus; and FIG. 20Athrough 20D is a definite example of handover information. The handoverinformation shown by FIG. 19 includes handover (HO) carry-outinformation for each data stream, target base transmission stationinformation, current base transmission station information and handovertiming information (i.e., handover carry-out time information).

The handover (HO) carry-out information for example indicates a datastream to be changed over among a plurality of simultaneouslytransmitted data streams, or a necessity of a changeover for each datastream. FIG. 20A is an actual example of handover carry-out information,storing data indicating a necessity of carrying out a handover for eachof a plurality of data streams, i.e., six data streams herein.

According to the “example 1” of FIG. 20A, the data streams to be changedover are the stream numbers “1”, “3”, “5” and “6” among the six datastreams. According to “example 2” of FIG. 20A, the data stream to bechanged over is “3” only. This information can be coded by “1” for datastreams to be changed over and “0” for those not to be changed over asexemplified by FIG. 20A, and stored in a predetermined area of FIG. 19as handover carry-out information. Alternatively, a stream number of adata stream to be changed over may be stored in a predetermined area ofFIG. 19 as handover information for example.

The target station information indicates a base transmission station 3which becomes the connecting station for the mobile station 2 aftercarrying out a handover. FIG. 20B is an actual example of target stationinformation. The “example 1” of FIG. 20B shows that the stream numbers1, 3, 5 and 6 are changed to the base transmission stations with thestation numbers “120”, “121”, “120” and “121”, respectively.Incidentally, the example 1 shows that the stream numbers 2 and 4 storezero (“0”) as the station numbers, indicating that no handover will becarried out. Likewise for the example 2, the data stream with the streamnumber 3 is changed over to the base transmission station 3 with thebase transmission station number “23”, while other data streams storezero (“0”), hence carrying out no handover.

The current station information indicates the base transmission station3 to which the mobile station 2 is connected prior to carrying out ahandover. FIG. 20C is an actual example of current station information.As shown by FIG. 20C, if a number other than zero (“0”) is stored as thenumber for a current base transmission station corresponding to a streamnumber, the base transmission station number of the base transmissionstation to which the mobile station 2 is connected prior to carrying outa handover is indicated. If zero (“0”) is stored, it indicates that ahandover is not carried out for the data stream of the correspondingstream number. In the example 1, the data shown by FIG. 20C indicatethat all four data streams for which a handover processing are about tobe carried out are connected to the base transmission station with thebase transmission station number “1”. Likewise in the example 2, thedata shown by FIG. 20C indicate that the number 3 data stream for whicha handover is about to be carried out is connected to the basetransmission station with the base transmission station number “1”.

The handover timing information stores an absolute time, a relative timeor a value in the unit of frames. FIG. 20D is an actual example ofhandover timing information. The handover timing information shown byFIG. 20D indicates how many frames after a notification of handoverinformation the handover is to be started. A value of zero (“0”) storedfor the data stream indicates that no handover is to be carried out. Inthe example 1 of FIG. 20D, handover is started at 20, 20, 18 and 22frames, respectively, for the data streams with the numbers 1, 3, 5 and6, respectively which are changed over by handovers. Likewise in theexample 2, a handover is started at ten frames for the number 3 datastream.

Note that the handover carry-out information is not limited to the abovedescribed structure shown by FIG. 19, but can be achieved by anystructure compiling the above described various pieces of informationfor each data stream for example. Or handover carry-out informationrelating only to a data stream for which a handover is to be carried outmay be indicated, while information relating to a data stream for whicha handover is not to be carried out may be omitted.

After paths are changed over for a predetermined data stream based onthe handover carry-out information, the buffer of the base transmissionstation (i.e., BTS_(3a) in the above described example), that is, thetransmission data unit 33 shown by FIG. 15 is left with un-transmitteddata of the changed over data stream st_(k). According to the handovermethod according to the present embodiment, the un-transmitted data istransmitted by another data stream. That is, handovers are not carriedout for all the data streams at once in the handover method according tothe present embodiment, by using a data stream for which a handover isnot carried out it is possible to transmit data until the dataaccumulated in the transmission data unit 33 at the current basetransmission station BTS_(3a) is transmitted.

FIG. 21 is a flow chart of transmission processing for accumulated datain a current base transmission station. The processing shown by FIG. 21is carried out if the step S6 of the process of judging a necessity-of ahandover shown by FIG. 17 judges that “there is a necessity of carryingout a handover”.

First, as the processing is started, the step S11 judges whether or nota handover is judged to be necessary for each data stream, and the stepS12 calculates the number of data streams (i.e., ST_rest) for which nohandover has been judged to be necessary. The step S13 judges whether ornot there is a data stream (i.e., ST_rest>0) for which a datatransmission continues from the current base transmission station aftercarrying out a handover.

If there is no continuation of a data transmission from the current basetransmission station after carrying out a handover, that is, if ahandover is carried out for all the data streams, the step proceeds toS14 to perform a handover for each data stream. The accumulated data aretransmitted to the mobile station 2 while the handovers are carried outfor data streams sequentially one after another. Note that theprocessing of the step S14 may be executed after all the accumulateddata in the base transmission station have been transmitted.

If a data transmission continues from the current base transmissionstation after carrying out a handover, that is, a data stream for whicha handover is not carried out exists, the processing proceeds to thestep S15 and calculates a data amount (i.e., Mdata) accumulated in thetransmission data unit 33 of the base transmission station. The step S16judges whether or not an accumulated data amount exists (i.e., isMdata>0?)

If an accumulated data does not exist in the base transmission station,the process proceeds to the step S17 to carry out a handover for therest of the data streams. If an accumulated data exists, the processproceeds to the step S18 to transmit until the accumulated data in thecurrent base transmission station BTS_(3a) is completely transmitted byusing a data stream for which a handover is not carried out.

Incidentally relating to a sequence of handover in the step S14, forexample, carrying out a handover in order of data stream received powerfrom the connecting base transmission station, being of relatively lowpower, allows a data transmission to be performed securely, therebymaking it possible to prevent the overall transmission speed fromdecreasing.

FIG. 22 illustrates a state of accumulated data at the base transmissionstation after carrying out a handover according to the presentembodiment. Data (i.e., data A), which has been transmitted from the RNCto the base transmission station BTS_(3a) prior to carrying out thehandover and is yet to be transmitted to the mobile station 2, is stillaccumulated in the buffer of the base transmission station BTS_(3a) atthe time of carrying out the handover. The handover method according tothe present embodiment makes it possible to transmit the un-transmitteddata A to the mobile station 2 by using an antenna with which a handoverhas not been carried out of the base transmission station BTS₃, by theprocessing shown by FIG. 21, even if the path for a data stream ischanged over to being via the base transmission station BTS_(3b).Incidentally, the data B, which is transmitted from the RNC to the basetransmission station BTS_(3b) at the time of changing over the path as aresult of the handover, is transmitted from the base transmissionstation BTS_(3b) to the mobile station 2.

When the data accumulated in the transmission data unit 33 of the basetransmission station BTS_(3a) is all transmitted to the mobile station2, the RNC cancels the connection with the base transmission stationBTS_(3a). Likewise in the target base transmission station BTS_(3i), ahandover is carried out based on the received power for each data streamaccording to the present embodiment.

The above described handover method is configured so that the RNCnotifies all the target base transmission station BTS_(3i), current basetransmission station BTS_(3a) and mobile station 2 of the handovercarry-out information as shown by FIG. 19, but is not limited as such.Meanwhile, the above described number of data streams, the basetransmission station number, et cetera, for which a handover is carriedout, are only examples and there is no intention to set an upper limit,et cetera. For example, a handover may be carried out for a target basetransmission station selected for each of every data stream or for aspecific data stream. Alternatively, a priority may be assignedaccording to a telecommunication environment for example, such ascarrying out a handover first for a specific data stream. The sameconsideration also applies to the below described embodiments.

As for the sequence of data streams for which handover is to be carriedout, it is possible to carry out all at once for all the data streamswhich are judged as the subject of handovers, or for each data streamsequentially one by one. In the case of carrying out a handover per datastream, it is possible to compare a received power from each target basetransmission station for each data stream and carry out a handover in asequence of the data stream with the lowest received power first, whichmakes it possible to transmit each data stream securely. Alternatively,it is possible to carry out a handover in a sequence of the data streamwith the highest received power first.

FIG. 23 is a control sequence chart of a handover judgment processing inthe case of the RNC judging a necessity of a handover. The descriptionis focused on a different point compared to the case of the RNC judginga necessity of a handover, as already described in reference to FIG. 18.

Having measured a received power of each data stream, the mobile station2 reports the measurement result by including it in a measurement resultmessage to the RNC. The measurement result message includes a streamnumber, a transmitting base transmission station, received powers, etcetera, for example.

Having received the measurement result notification, the RNC judges anecessity of a handover for each data stream based on the informationincluded therein as in the case of the mobile station 2 judging it basedon the received power as shown by FIG. 18. For the data stream judged asa handover being necessary in the aforementioned judgment, anotification of handover information is transmitted to the target basetransmission station, the current base transmission station and themobile station 2, and then a handover processing is carried out in thesame way as the previously described method in the ensuing process.

The handover judgment processing carried out by the mobile station 2 andthe RNC, respectively, in the process shown by FIGS. 18 and 23, may beperformed by a base transmission station 3. If the base transmissionstation 3 judges a necessity of a handover, the sequence thereof is forit to receive information including a received power for each datastream measured at the mobile station 2 therefrom. As the basetransmission station 3 judges a necessity of a handover based on thereceived information, thus judging a handover being necessary, thehandover control information shown by FIGS. 19 and 20A through 20D istransmitted as a notification to the RNC, the mobile station 2 and thetarget base transmission station. Then a handover per data stream iscarried out based on the notified handover control information.

Note that the above description utilizes a received power for judging anecessity of a handover, but it is not limited as such. For example, areceived field intensity or a received quality such as SIR (i.e., asignal to interference power ratio) may be used instead. The followingdescription also utilizes a received power for judging a necessity of ahandover, but it is also possible to carry out the same handover byusing a received field intensity or a received quality.

The above described embodiment exemplifies a transmission from the basetransmission station 3 to the mobile station 2, but it is not limited assuch. For example, it is possible to apply the above described handovermethod to a transmission from a mobile station 2 to a base transmissionstation 3, or that between mobile stations 2. Note that the followingdescription is the same in terms of not being limited to thetransmission from the base transmission station 3 to the mobile station2.

Second Embodiment

FIG. 24 shows a comprisal of the mobile station 2 carrying out ahandover according to the second embodiment. The description is of adifferent point as compared to the comprisal of the mobile station 2according to the first embodiment shown by FIG. 13. Note that the basetransmission station 3 for carrying out a handover according to thepresent embodiment is the same as that of the base transmission stationaccording to the first embodiment as shown by FIG. 15, and therefore adescription thereof is omitted here.

The mobile station 2 according to the second embodiment shown by FIG. 24differs from the mobile station 2 according to the first embodimentwhere the former comprises received power measurement units 27 (27A, 27Band 27C) for measuring a received power of a received pilot signalreceived by way of each antenna Rx. The received power measurement units27A, 27B and 27C measure the received powers of signals received by wayof respective antennas and provide the measurement results to thehandover judgment unit 23 which then calculates a sum of the receivedpower (a total received power) provided by each received powermeasurement unit 27 and determines a target base transmission station.The process of determining which data stream to carry out a handover foramong a plurality thereof is decided based on the measurement result ofthe stream received power measurement unit 22, which is the same as thefirst embodiment.

FIG. 25 is a flow chart of a handover processing according to thepresent embodiment. The processing shown by FIG. 25 is carried out everytime the mobile station 2 receives a pilot signal, transmitted from thebase transmission station 3 at a predetermined time interval. Thefollowing description is of a handover method according to the presentembodiment referring to FIG. 25.

First, the step S21 is for the received power measurement unit 27 tomeasure a total received power from the currently connected basetransmission station BTS_(3a) and the nearby base transmission stationsBTS_(3b) through BTS₃. The step S22 is to determine a base transmissionstation BTS_(3h) with the maximum received power by comparing the valuesof each total received power. The step S23 is to compare the totalreceived power P_(bts3h) from the base transmission station BTS_(3h)with the total received power P_(bts3a) from the currently connectedbase transmission station BTS_(3a) and, if the P_(bts3h) is larger thanP_(bts3a), the judgment is that it is necessary to carry out a handover.

In the step S24, the stream received power measurement unit 22 measuresa received power for each data stream from the currently connected basetransmission station BTS_(3a) and the base transmission station BTS_(3h)which is judged as the target station in the step S23. The step S25 isto compare the received power from the base transmission stationBTS_(3a) with that from the base transmission station BTS_(3h) for eachdata stream; and, if the received power from the base transmissionstation BTS_(3h) is the largest, then the step S26 is to judge that ahandover for the data stream is necessary. The step 27 is to report thehandover control information relating to the base transmission stationBTS_(3h) and data stream to the RNC via the base transmission stationBTS_(3a), and the step S28 is to carry out a handover based on thereport to end the processing.

In the steps S25 and S26, the process of selecting a data stream to havea handover carried out by comparing the received power is the same as aseries of processes according to the first embodiment as shown by FIG.17. The difference lies in that a target base transmission station thatis common to a data stream, for which a handover is carried out, isjudged based on the value of the total received power.

As described above, according to the present embodiment, a handover iscarried out for a data stream for which a handover is necessary and fora base transmission station with a larger received power than thecurrently connected base transmission station. The benefit is that ahandover is carried out to the common base transmission station for eachdata stream instead of carrying out a handover to a different basetransmission station therefor, in addition to the benefit of thehandover method according to the above described embodiment. That is,there is no possibility of connections between three or more basetransmission stations and a mobile station at the time of a handover perstream, and therefore an improved efficiency of data transmission can beexpected.

Third Embodiment

A handover method according to this embodiment is the same as in thecase of the second embodiment where the path for a data stream to carryout a handover is switched to a common target base transmission station,while the difference from the second embodiment is that a target stationis determined based on statistical data of the optimal target basetransmission station for each data stream. Since the comprisals of themobile station 2 and base transmission station 3 according to thepresent embodiment are the same as in the case of the first embodiment,descriptions of them are omitted in the following description of ahandover method according to the present embodiment.

FIG. 26 is a flow chart of a handover processing according to the thirdembodiment. The processing shown by FIG. 26 is executed at the time ofreceiving a pilot signal from the base transmission station 3 as in thecase of the processes shown by FIGS. 17 and 25. Note that the flow chartshown by FIG. 26 is for the case of the number of simultaneouslytransmitted data streams being “m”.

First, the step S31 is to initialize, to zero (“0”), a stream number kfor identifying each data stream. The step S32 is to increment thestream number k and the step S33 is to measure a received powerP_(bts3a) _(—) _(stk) of the data stream by the stream number k from thecurrently connected base transmission station BTS_(3a). The step S34 isto measure the received powers P_(bts3b) _(—) _(stk) through P_(bts3n)_(—) _(stk) of the data stream with the stream number k from the nearbybase transmission stations BTS_(3b) through BTS_(3n), and the step S35is to calculate the maximum received power P_(bts3h) _(—) _(stk) amongthe P_(bts3a) _(—) _(stk) through P_(bts3n) _(—) _(stk).

The step S36 is to compare the maximum received power P_(bts3h) _(—)_(stk) with the received power P_(bts3a) _(—) _(stk) from the currentlyconnected base transmission station BTS_(3a). If the value of thereceived power P_(bts3a) _(—) _(stk) exceeds that of the maximumreceived power P_(bts3h) _(—) _(stk), the process proceeds to the stepS37 judging that no handover is necessary for the data stream and in thestep S38 storing the currently connected base transmission station“BTS_(3a)” as the selected target base transmission station. Havingstored it, the process proceeds to the step S41. Here, the selectedtarget base transmission station means a candidate base transmissionstation for a common target base transmission station for the datastream at the time of carrying out a handover.

If the maximum received power P_(bts3h) _(—) _(stk) exceeds the receivedpower P_(bts3a) _(—) _(stk) from the currently connected BTS_(3a), thenthe process proceeds from the step S36 to the step S39, judging that ahandover is necessary. The step S40 is to store the base transmissionstation “BTS_(3h)” with the maximum received power as the selectedtarget base transmission station, and the process proceeds to the stepS41.

The step S41 is to judge whether or not the processes in the steps S33through the step S38, or through the step S40 are completed for all thedata streams and repeat the processes in the step S32 and thereafteruntil the processes for the data stream with the stream number k isequal to m (k=m).

FIG. 27A exemplifies a selected target base transmission station storedin the steps S38 and S40. The table stores a base transmission stationwith the maximum received power, as the selected base transmissionstation, for each data stream correlated therewith.

The step S42 is to determine a target base transmission station byreferring to the selected target base transmission station for each datastream. The present embodiment is configured to determine the mostselected base transmission station as the target base transmissionstation. For example, the data table shown by FIG. 27A indicates thatthe base transmission station BTS_(3b) is stored as the most “selectedbase transmission station”, and therefore the base transmission stationBTS_(3b) is selected as the target base transmission station.

The processes in the steps S43 and S44 correspond to, and are the sameas, those in the steps S27 and S28 shown by FIG. 25, and thereforedescriptions thereof are omitted herein. FIG. 27B exemplifies a receivedpower from the base transmission station determined in the previous stepas the target base transmission station for each stream. The step S44 isenabled to carry out a handover in the sequence of the data stream withthe highest, or the lowest, received power from among the receivedpowers from those base transmission stations for which handovers are tobe actually carried out as shown by FIG. 27B.

According to the present embodiment as described above, a common targetbase transmission station is selected instead of the different targetbase transmission stations for each of the data streams, with the eachtarget base transmission station being selected based on the receivedpower of each data stream. That is, a handover is carried out to thecommon target base transmission station according to the state of a datastream therefor to be handed over to.

Incidentally, as to which data stream to carry out a handover for fromamong those data streams for which a handover is judged to be necessary,the data stream with the highest received power, or the lowest, maybegiven priority for example. In another method, subtracting the receivedpower P_(bts3a) _(—) _(stk), of the currently connected basetransmission station, from the received power P_(bts3x) _(—) _(stk) of aadjacent base transmission station BTS_(3x), and determining thesequence of carrying out a handover based on the difference of thereceived powers P_(diff) _(—) _(stk)=P_(bts3x) _(—) _(stk)−P_(bts3a)_(—) _(stk) may be possible.

Fourth Embodiment

A handover method according to this embodiment differs from the abovedescribed embodiment in that the former sets a threshold value for eachdata stream in advance and judges a handover is necessary when thedifference of a received power from the currently connected basetransmission station and that from a adjacent base transmission stationexceeds the threshold value.

FIG. 28 is a flow chart of a handover processing according to the fourthembodiment. The above described threshold set for judging a handover isdefined as P_(th). The timing for starting the processing shown by FIG.28 is at the time of receiving a pilot signal as with the processesshown by FIGS. 17, 25 and 26. The processes in the steps S51 through S55and S56 correspond to the steps Si through S5 and S7 shown by FIG. 17,carrying out the same processes, respectively, and thereforedescriptions thereof are omitted herein.

The step S57 is to calculate the difference P_(diff) _(—) _(stk) betweenreceived powers as noted in the description of the third embodiment. Thestep S58 is to compare the size of the difference P_(diff) _(—) _(stk)and the threshold value P_(th). If the threshold value P_(th) isexceeded by the difference P_(diff) _(—) _(stk), the process proceeds tothe step S59, judging that a handover is necessary, while if thedifference P_(diff) _(—) _(stk) is at the threshold value P_(th) orless, then the process proceeds to the step S56, judging that a handoveris not necessary, followed by ending the processing.

According to the handover method of the present embodiment, a handoveris judged as being unnecessary until the difference between the receivedpower from the currently connected base transmission station and thereceived powers from the adjacent base transmission stations exceeds apredetermined magnitude. This makes it possible to prevent handoversfrom being carried out excessively frequently, in addition to thebenefit of the handover according to the above described embodiments.

Note that the above described example assumes a common threshold valuebeing set, but the same benefit is obtained as the handover methodaccording to the present embodiment if a different threshold valueP_(th(k)) is set for each stream.

Fifth Embodiment

A handover method according to this embodiment is configured to performdiversity transmission for a data stream from a base transmissionstation after carrying out either of the handover methods according tothe above described embodiments by taking advantage of the fact that thenumber of data streams is reduced in the aforementioned basetransmission station (i.e., a part of the resources for transmittingdata (e.g., a part of the plurality of antennas) assuming a state of notbeing used). The following description of the handover method accordingto the present embodiment, refers to the accompanying drawing.

FIG. 29 illustrates a handover method according to the fifth embodiment.As shown by FIG. 29(a), the base transmission station BTS_(3a) transmitsthree data streams st₁, st₂ and st₃ simultaneously to the mobile station2 prior to a handover. Assuming that, having carried out a handover byeither of the above described handover methods according to the firstthrough fourth embodiments, the data stream st₂ is changed over to atransmission via the adjacent base transmission station BTS_(3b).

The antenna Tx₂ of the base transmission station BTS_(3a) is unused as aresult of the change over to the transmission via the adjacent basetransmission station BTS_(3b). As shown by FIG. 29(b), a diversitytransmission is performed for a data stream for which a handover is notcarried out at the base transmission station BTS_(3a) by using theantenna Tx₂ which has become unused as a result of the handover. In theexample shown by FIG. 29(b), a diversity transmission is performed forthe data stream st₁ which used to be transmitted by way of the antennaTx₁. By the diversity transmission, the same data stream st₁ istransmitted to the mobile station 2 by way of the antennas Tx₁ and Tx₂of the base transmission station BTS_(3a). This configuration makes itpossible to improve the transmission quality of the data stream st₁ atthe time of a handover according to the above described embodiment, andalso increase the transmission speed as a result of improving thetransmission quality.

Meanwhile, as for a selection of a data stream as the subject of thediversity transmission, a configuration may be to select a data streamwith a large received power, or with a small received power, at themobile station 2. Or, if there is an attribute of transmission data,that is, a QoS (quality of service) such as the maximum delay time andthe desired transmission quality, for each data stream is different,then the selection may be based on the attribute.

Sixth Embodiment

This embodiment changes over the method for a data transmission from aMIMO transmission to a diversity transmission according to atelecommunication environment, et cetera.

FIG. 30 is a conceptual diagram of a system configuration according tothe sixth embodiment. The area A is a cell area relatively close to thebase transmission station 3, while the area B is a cell area relativelyfar from a base transmission station. If the mobile station 2 is in thearea A, a received power therein is large, thus good telecommunicationquality is anticipated. With the mobile station 2 moving to the area B,the received power at the mobile station 2 decreases, hence a decreasedtelecommunication quality is anticipated. The present embodiment isconfigured to change over from a data telecommunication by the MIMO tothat by a diversity transmission per stream one after another based onthe received powers, et cetera, for example, in an environment where adecreased telecommunication quality is anticipated.

FIG. 31 is a flowchart of a transmission method changeover processingaccording to the present embodiment. The processing shown by FIG. 31 isstarted at the timing of the mobile station 2 receiving a pilot signalfrom the base transmission station 3 as in the case of the abovedescribed embodiments.

First, the step S61 is to measure a received power from the currentlyconnected base transmission station (BTS_(3a)), which measures a total(defining P_(bts3a)) of the power received by each antenna of the mobilestation 2. The next step S62 is to compare a predetermined thresholdP_(bts3a) _(—) _(th) which is set for the base transmission stationBTS_(3a) with the received power P_(bts3a) If the received powerP_(bts3a) is the same as, or greater than, the threshold value P_(bts3a)_(—) _(th), the process proceeds to the step S63 and continues thetelecommunication by the MIMO, ending the processing. If the receivedpower P_(bts3a) is smaller than the threshold value P_(bts3a) _(—)_(th), the process proceeds to the step S64 and initialize the streamnumber k to zero (“0”), followed by proceeding to the step S65. Theprocesses in the steps S65 through S67 are repeated until the receivedpowers P_(bts3a) _(—) _(stk) from the currently connected basetransmission station BTS_(3a) for the data stream with the stream numberk are measured for all data streams. FIG. 31 exemplifies the case of them-number of data streams.

The step S68 is to compare the received powers of data streams, therebyselecting a data stream to be changed from the MIMO telecommunicationover to the diversity transmission; and the step S69 is to change theselected data stream to the telecommunication by the diversitytransmission, followed by ending the processing.

Note that the above described example judges which of the MIMO ordiversity transmission to select for a transmission based on themagnitude of the received power, however the configuration is notlimited as such. For example, the judgment may be made in considerationof both the received power and a reception quality such as SIR.

Also, in the step S62 the judgment of whether or not the transmissionmethod for the data stream is to be changed from the transmission by theMIMO to that of the diversity transmission is based on the measurementof a received power independent of a stream, but the configuration isnot limited as such. For example, a threshold value P_(th(k)) may beprepared for a data stream with the stream number k and the judgment maybe made based on a magnitude relationship with a received powerP_(bts3a) _(—) _(stk) corresponding to the threshold P_(th(k)).

As described above, the present embodiment makes it possible to transmita data stream by a telecommunication method according to thetelecommunication environment even in the case of a reduced receivedpower occurring, hence contributing to maintaining a telecommunicationquality.

1. A wireless telecommunication system for transmitting a plurality ofstreams from a transmission apparatus comprising a plurality of antennasto a receiving apparatus, comprising: a transmission unit, beingequipped in the transmission apparatus, for transmitting a plurality ofstreams; a receiving unit, being equipped in the receiving apparatus,for receiving each stream transmitted from the transmission unit; ameasurement unit for measuring respectively a reception quality or areceived power from the transmission unit relating to each stream; and achangeover unit for carrying out a handover for each of the streamsaccording to a measurement result by the measurement unit.
 2. Thewireless telecommunication system according to claim 1, furthercomprising: a judgment unit for judging whether or not it is necessaryto carry out a handover by comparing a reception quality or receivedpower relating to a stream from a first transmission unit in a firsttransmission apparatus being used for a telecommunication with areception quality or received power, respectively, relating to a streamfrom a second transmission unit in a second transmission apparatus,wherein said changeover unit changes a transmission unit used for atelecommunication for a stream from the first transmission unit to thesecond transmission unit, if it is judged to necessary to carry out thehandover.
 3. The wireless telecommunication system according to claim 2,wherein said judgment unit judges that it is necessary to carry out ahandover if the reception quality or received power relating to thestream from said second transmission unit becomes a predeterminedthreshold value or greater, respectively, compared to the receptionquality or received power relating to the stream from said firsttransmission unit.
 4. The wireless telecommunication system according toclaim 2, wherein said changeover unit changes either in ascending ordescending order of a reception quality or received power when carryingout a handover relating to a plurality of streams.
 5. The wirelesstelecommunication system according to claim 2, wherein each streamcomprises identifier information for identifying the stream from otherstreams, and said changeover unit recognizes a stream for which saidfirst transmission unit is to be changed over to said secondtransmission unit based on the identifier information.
 6. The wirelesstelecommunication system according to claim 2, further comprising: adiversity unit for carrying out a diversity transmission by using saidfirst transmission unit and a third transmission unit equipped in saidfirst transmission apparatus after the transmission unit which is totransmit the stream is changed to said second transmission unit by saidchangeover unit.
 7. The wireless telecommunication system according toclaim 2, further comprising a total received power measurement unit formeasuring a total received power of each transmission apparatus, whereinsaid judgment unit judges that it is necessary to carry out a handoverrelating to a transmission unit for said transmission apparatus if thetotal received power from said second transmission apparatus exceeds thetotal received power from the first transmission apparatus.
 8. Thewireless telecommunication system according to claim 2, wherein saidchangeover unit carries out the handover for each stream to atransmission apparatus most selected as a handover destination for astream for which it is necessary to carry out the handover.
 9. Awireless telecommunication system for transmitting a plurality ofstreams from a transmission apparatus comprising a plurality of antennasto a receiving apparatus, comprising: a transmission unit, beingequipped in the transmission apparatus, for transmitting a plurality ofstreams; a receiving unit, being equipped in the receiving apparatus,for receiving each stream transmitted from the transmission unit; ameasurement unit for measuring respectively a reception quality or areceived power from the transmission unit relating to each stream; and achangeover unit for changing between a state of each transmissionantenna transmitting a mutually different stream for each of the streamsand a state of transmitting by a diversity transmission according to ameasurement result by the measurement unit.
 10. A transmitter comprisinga plurality of antennas for transmitting a plurality of streams,comprising: a transmission unit for transmitting a plurality of streams;and a changeover unit for carrying out a handover for each streamaccording to respective measurement results of a reception quality orreceived power from the transmission unit relating to each stream. 11.The transmitter according to claim 10, further comprising: anaccumulation unit for accumulating data to be transmitted, wherein aleftover transmission unit transmits data accumulated in theaccumulation unit after a transmission unit is made to assume an unusedstate by said changeover unit.
 12. A receiver for receiving a pluralityof data streams, comprising: a receiving unit for receiving a pluralityof streams transmitted from a first and second transmitters; ameasurement unit for respectively measuring a reception quality orreceived power from the transmitter relating to each stream; and asignal reproduction unit for reproducing a signal from one or morestreams transmitted from the first transmitter and one or more streamstransmitted from the second transmitter if it is judged as necessary tocarry out a handover to the second transmitter for a part of streamsamong a plurality thereof transmitted from the first transmitter basedon a measurement result by the measurement unit.